This application is based on French Patent Application No. 01 14 792 filed Nov. 15, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to a self-synchronized synchronous rectifier, in particular a self-synchronized synchronous rectifier used in an AC/DC or DC/DC converter.
The invention relates to a symmetrical or asymmetrical coupled winding synchronized or self-synchronized forward energy transfer synchronous rectifier. In the remainder of the text, the expression xe2x80x9cself-synchronized synchronous rectifierxe2x80x9d also refers to a xe2x80x9ccoupled winding synchronized synchronous rectifierxe2x80x9d.
2. Description of Related Art
Asymmetrical converter systems comprising an initial voltage source feeding a transformer primary connected in series with a main switch are known in the art. The transformer secondary is connected in cascade with a self-synchronized synchronous rectifier and a filter. The output of the filter delivers a controlled DC voltage to a load. In this type of converter system, the self-synchronized synchronous rectifier has the following functions:
delivering to the load, via the filter, the energy transferred by the transformer in the periods of conduction of the main switch, and
blocking the transfer in the periods of non-conduction of the main switch, the load being supplied with power by the coil of the filter during periods of non-conduction of the main switch.
An asymmetrical self-synchronized synchronous rectifier comprises two MOSFETs adapted to provide the above two functions to reduce the losses of the rectifier. For example, an asymmetrical self-synchronized synchronous rectifier includes:
first and second rectifier outputs,
a first MOSFET connected between the first transformer secondary end and the first rectifier output and having a gate connection to the second end of the transformer secondary, and
a second MOSFET connected between the first rectifier output and the second rectifier output and having a gate connection to the first end of the transformer secondary.
The voltage at the secondary of the transformer controls the two MOSFETs.
For economic reasons, and to obtain a small overall size, manufacturers wish to develop converters offering, in the same product, a wide range of input voltage and varied output voltages. This implies a high operating frequency and voltage variations at the secondary of the transformer proportional to those at the input. The voltage at the secondary of the transformer also constitutes the gate signal of the MOSFETs of the rectifier. The voltage that can be applied to the gates of the MOSFETs is limited. If too high a voltage is applied to the gate of a MOSFET, it may be destroyed or generate unacceptable switching losses. These losses are proportional to the switching frequency.
To protect MOSFETs against gate overvoltages, it is known in the art for the gate connections to be in series with a passive voltage divider bridge. However, the presence of the voltage divider causes high losses if the voltage at the secondary of the transformer is too low or too high. This solution also has the drawback of significant switching losses associated with the reverse bias voltages of the synchronous rectifier MOSFETs.
A self-synchronized synchronous rectifier can be envisaged having gate protection allowing a wide variation in the input voltage combined with optimum performance in terms of output current and voltage.
A prior art solution provides an asymmetrical self-synchronized synchronous rectifier connected between a transformer secondary winding, having first and second transformer ends and an LC filter having first and second filter inputs, said asymmetrical self-synchronized synchronous rectifier including:
first and second rectifier inputs respectively connected to the first and second transformer ends,
a first rectifier output and a second rectifier output which is connected to the second rectifier input and to an input filter,
a forward MOSFET connected between the first rectifier input and the first rectifier output and having a gate connection connected to the second rectifier input through a gate protection circuit, and
a freewheel MOSFET connected between the first rectifier output and the second rectifier output and having a gate connection connected to the first rectifier input through a gate protection circuit.
Here, each gate protection circuit constitutes, with the intrinsic or additional gate-source capacitance of the corresponding MOSFET, a controlled divider bridge, and the rectifier includes a control device receiving an input signal proportional to the input voltage of the rectifier and producing output signals for controlling the gate protection circuits. It is usual to provide low impedances in series with the gate protection circuit, which does not alter the general principle described.
The advantages of this kind of solution result from dynamic control of the gate voltage, which enables the voltage divider bridge to be short-circuited or not, as a function of the voltage of the secondary of the transformer or any voltage equivalent to the input voltage. Accordingly, regardless of the amplitude of the input voltage of the self-synchronized synchronous rectifier, the gate voltages of the MOSFETs are optimized to limit losses and conserve an optimum switching dynamic. Accordingly, for high variations of input voltage, and for the same output voltage and the same volume, a converter including a self-synchronized MOSFET rectifier can pass more power.
However, although it is effective in reducing switching losses and conduction losses, this solution proves to be bulky and costly as soon as the range of input voltage is normal, and also for very low output voltages.
The present invention solves these problems, limiting the reverse voltage of the synchronous rectifier MOSFETs, in the phase in which they are turned off, to the conduction threshold voltage of a limiter, and at the same time assuring correct biasing of the gates in the respective phases in which the MOSFETs are turned on.
To this end, a power MOSFET gate protection circuit in accordance with the invention includes a limiter having a low voltage conduction threshold controlled by its own wiring and including a first portion connected in parallel with a divider and adapted to be connected to the gate of an associated power MOSFET and a second portion adapted to be connected to the source of the associated power MOSFET.
In a first embodiment of the invention the limiter is a transistor controlled by its gate-source wiring, its source being adapted to be connected to the gate of the associated power MOSFET and the gate being adapted to be connected to the source of the associated power MOSFET.
The transistor is advantageously an MOS transistor.
In a second embodiment of the invention the limiter comprises a first diode connected in parallel with the divider component and adapted to be connected to the gate of the associated power MOSFET and a second diode adapted to be connected in parallel between the source and the gate of the associated power MOSFET.
The first diode is preferably a signal diode and the second diode is preferably a protection diode.
The invention also provides a self-synchronized synchronous rectifier adapted to be connected between a transformer secondary winding having first and second transformer ends and an LC filter having first and second filter inputs, the self-synchronized synchronous rectifier including:
first and second rectifier inputs respectively connected to the first and second transformer ends,
a first rectifier output and a second rectifier output which is connected to the second rectifier input,
a forward MOSFET, or any voltage-controlled component, connected between the first rectifier input and the first rectifier output and having a gate connection, and
a freewheel MOSFET, or any voltage-controlled component, connected between the first rectifier output and the second rectifier output and having a gate connection,
wherein at least one of the gate connections is connected to a rectifier input via a gate protection circuit which includes a limiter having a low voltage conduction threshold controlled by its own wiring, a first portion of the limiter is connected in parallel with a divider component and adapted to be connected to the gate of the associated power MOSFET and a second portion of the limiter is adapted to be connected to the source of the associated power MOSFET.
In a first embodiment of the invention the limiter is a transistor controlled by its gate-source wiring, the source is adapted to be connected to the gate of the associated power MOSFET, and the gate is adapted to be connected to the source of the associated power MOSFET.
The transistor is advantageously an MOS transistor.
In a second embodiment of the invention the limiter comprises a first diode connected in parallel with the divider and adapted to be connected to the gate of the associated power MOSFET and a second diode adapted to be connected in parallel between the source and the gate of the associated power MOSFET.
The first diode is preferably a signal diode and the second diode is preferably a protection diode.
The wiring of the limiter advantageously includes series impedances.
The protection circuit can be connected to the rectifier via a second divider in series with the first. The second divider can provide active gate protection, as shown in FIG. 3A.
The rectifier can be wired to a center-tapped transformer secondary.
The invention is described in more detail hereinafter with the assistance of figures showing a preferred embodiment of the invention.